FCE Ultra is an emulator of the original (8-bit) Nintendo Entertainment
System (NES) released under the GNU General Public License (GPL). It
includes the following features (and many more):
* fully customizable color palette rendering engine
* complete sound emulation (all five channels)
* support for up to four joystick controllers
* zapper emulation for the mouse
* GameGenie emulation
* accepts compressed (PKZIP, gzip) ROM images
* TCP/IP network play

/usr/share/doc/fceu/Documentation/tech/README.now

Many(possibly all) of these documents contain flaws or are incomplete, so
don't pull out your hair if there are inconsistencies between the documents,
what's in FCE Ultra, and what you observe. That's not to say that FCE Ultra
doesn't have its share of (emulation) flaws, though...
For many more NES-related documents, try http://nesdev.parodius.com

/usr/share/doc/fceu/Documentation/tech/README.sound

Sound information is in the "cpu" subdirectory, due to the intimate
relationship between the sound circuitry and the cpu.